do you have them laying down for them to get to at anytime or mixed in the food? Ducks don't really need calcuim, even if she is a layer, to much will cause issues. They need about 2% and I believe the food supplies more then enough.
Is she acting any differently? Such as staggering or falling or being slower then the norm. What about swim time?? Does she act normal like doing the bobbing thing to clean herself or flapping her wings in the water to give you a bath too. Is she doing anything out of the norm??
Crack corn should be giving as a treat not a daily in take. Cut back on the crack corn. They tend to get full on it and will refuse or slow down there daily feed.